The present invention is based, in general, on the field of agriculture and the processing of harvested crops. Vehicles designed to pick up and process crops—self-propelled agricultural harvesting machines in particular—are used for this purpose. The self-propelled agricultural harvesting machines are typically combine harvesters, forage harvesters, and all types of lifters that are equipped with electronic control devices for drive units. Some of the drive units act on the ground drive, which is composed of an all-wheel drive that acts on all of the wheels, and/or a track roller unit in contact with the ground.
The front wheels of a harvesting machine may be composed of a track roller unit with two large guide wheels. The wheels are driven by a main engine, which is designed, e.g., as an internal combustion engine, by a hydraulic pump, and at least one hydraulic motor, which is connected with the hydraulic pump, converts the hydraulic energy into mechanical work, and drives at least one wheel that is engaged with the ground. Electronic controls and actuators are used to control and regulate the hydraulic motors, which, as drive units, drive front and rear wheels using a hydrodynamic transfer of force. Control units that control the drive torque at the wheels are used to obtain good traction and minimize the slippage of the wheels on the ground. Sensors that process the signals from the control devices are used at particular points in the self-propelled harvesting machine to control the drive torques.
Various systems are known from the related art for increasing the traction of vehicles and reducing the tire slip between the drive wheel and the ground.
One such system is an all-wheel drive, with which—in complete contrast to front-wheel drive or rear-wheel drive—the engine force of a vehicle acts on all wheels in contact with the ground, in order to ensure that the vehicle may travel across rough terrain at all. All-wheel drive increases traction by distributing the drive torques and improving driving stability, and it is adequately known from the related art. With vehicles with permanent all-wheel drive, engine power is transferred constantly to all four wheels, and differentials ensure that the rotational speed is fully equalized and that power is not lost. To prevent strain from occurring in the drive train, an additional central differential is installed between the front and rear axles. The disadvantage of this is that, if any single wheel or axle has inadequate traction or no traction, the amount of drive torque that may be transferred by this wheel or axle is limited. As a result, in the extreme case, the vehicle becomes unable to move under its own force.
Assistance in these cases is provided by a special transmission and an electronic control, referred to as VDC (vehicle dynamic control). The special transmission compensates for the unequal weight distribution between the axles of the agricultural machines by dividing the force between the front and rear axles at a ratio of, e.g., 35:65. The electronic control compares the wheel speeds and steering angle specified by the drive, to detect overcontrol or undercontrol and to modify the force distribution such that the vehicle retains a neutral self-steering effect.
Publication DE 199 21 856 A1 discloses a hydrostatic-mechanical all-wheel drive for multiple-axle vehicles that makes it possible to distribute the ground drive output to the driving axles with consideration for the traction that exists between the vehicle tires and the ground. The drive presented ensures automatic, stepless adjustment of the speed ratios, but it may also be supported via regulator intervention depending on the steering angle, thereby resulting in torque being distributed to the drive axles by changing the rotational speed ratios. Strain is thereby prevented, as described previously in the related art, even when driving through tight turns. Output is distributed such that wheel slip is kept the same between all driven wheels, even when the loads on the vehicle axles change.
The disadvantage of this drive system is that the control and regulating device measures only a few parameters of the vehicle. As a result, power distribution, which is carried out by adjusting the hydraulic motors, does not take place under all driving and harvesting conditions.
The aforementioned control and regulating device is referred to as ASR (anti-slip regulation) or TCS (traction control system), and is known from the related art. Anti-slip regulation ensures that the wheels do not spin when they are accelerated. When giving too much gas at start-up or if the terrain is poor and static friction is minimal—circumstances which occur very often in agriculture—one or more wheels may spin, and the vehicle becomes unstable. The spinning of one or more wheels on the ground is referred to as tire slip. To ensure a maximum transfer of friction force between the tires and the various ground conditions in fields, and in various weather conditions, ASR is used to prevent wheels from spinning and/or to prevent undercontrol and overcontrol of the vehicle. If there is a risk of serious slip of the drive wheels, the drive torque is regulated via targeted intervention by the braking system and/or engine management system.
The closed-loop control system, which receives its information, e.g., from the ABS wheel speed sensors, therefore ensures traction and driving stability during the acceleration phase on a straight path, even when driving around turns. ABS stands for “antilock braking system” and also improves the driving safety of agricultural vehicles. It functions mainly in certain situations of hard braking by regulating the braking pressure in short intervals to counteract the tendency of the wheels to lock up. ABS is also capable of controlling the braking behavior of each individual wheel in a nearly optimal manner. The ESP (electronic stability program) also prevents slip and the undercontrol or overcontrol of a vehicle using electronic sensors, the signals of which are evaluated and processed in a system, by braking individual wheels in a specific manner.
The system ascertains the driving behavior in this manner and intervenes when a deviation from the driver's information setting is determined. The change in steering angle is also taken into consideration. Hydraulic motors may be braked by changing the volumetric flow rate of the hydraulic fluid.
The ESP, ASR, and ABS control systems do not meet the requirements placed on drive wheels of motor vehicles, however, in particular on agricultural harvesting machines, for attaining optimal traction and limiting it such that tire slip may be adequately prevented under specific harvesting conditions.
Publication EP 1 350 658 B1 makes known a control device for an hydraulic all-wheel drive that controls actuators using signals generated by sensors (a speed sensor and a pressure sensor). The actuators influence two drivable axles or hydraulic motors assigned to the axles by changing the intake volume, in order to obtain optimal traction at the drive wheels. This type of control not only takes the aforementioned driving behavior into account, but also the load distribution on the front and rear axles when driving up or down hills, thereby making it possible to prevent the familiar “back-spin” effect. When the loads on the axles change when driving up or down hill, this is compensated for by changing the drive output at the wheels. The axle loads are not actually measured, however. Instead, the operating state of the hydraulic motor is ascertained via a pressure sensor, the signal of which contains information about the pressure difference between the inlet and outlet channels in the hydraulic motor, and via the speed sensor signal. Based on the signal from the pressure sensor, it may be determined whether the wheel is driving the vehicle, or whether the wheel is spinning. The disadvantage of this system is that the slip of one or more wheels is not detected at an early stage, and it cannot be counteracted until after slip has already occurred.
Publication EP 1 232 682 B1 describes an electronic engine control for an internal combustion engine. Performance curves for controlling the internal combustion engine are stored in the control. The performance curves contain information about engine output as a function of various parameters, e.g., engine speed, temperature, the drive of the attachments and devices, etc. According to the present invention, the control contains machine parameters of crop material-gathering devices. When a different crop material-gathering device is used, this is detected via sensors. A sensor may be a switch in the driver's cab, which the driver may operate. When a different crop material pick-up device is used, the performance curve of the internal combustion engine is changed in the engine control. The change to the performance curve results in the output of the internal combustion engine being regulated up or down. The performance of the internal combustion engine is therefore regulated depending on the front attachment that is used. The drive torque of the individual drive wheels at the hydraulic motors is not regulated, however.